Process for fracturing an underground formation



J. L. HUITT 3,

PROCESS FOR FRACTURING AN UNDERGROUND FORMATION Oct. 12, 1965 INVENTOR.

Filed June 14, 1962 ATTORNEY United States Patent 3,211,221 PROCESS FOR FRACTURING AN UNDERGROUND FORMATION Jimmie L. Huitt, Shaler Township, Allegheny County,

Pa., assignor to Gulf Research & Development Company, Pittsburgh, Pa., a corporation of Delaware Filed June 14, 1962, Ser. No. 205,165 2 Claims. (Cl. 16642) This application is a continuation-in-part of United States application for Letters Patent Serial No. 19,791, filed April 4, 1960, now abandoned.

This invention relates to the treatment of earth formations penetrated by wells such as oil wells and the like, and more particularly to a method for positioning and orienting artificial fractures in earth formations.

It is well known in the art of well stimulation to inject a fluid into an earth formation and to apply pressure to the fluid to create artificial fractures in the formation. The creation of artificial fractures in fluid bearing earth formations facilitates drainage of fluids present in such formations into a well from which the desired fluids can be recovered. In hydraulic fracturing operations a fracturing liquid, generally a viscous or otherwise non-penetrating liquid, is forced down the Well and into the formation under sufiicient hydraulic pressure to overcome the tensile strength of the formation, that is, to break down the formation thereby causing a crack to be produced in the formation. The fracturing fluid upon being forced into the induced crack extends the fracture substantial distances into the formation. Usually a propping agent is suspended in fluid displaced into the fracture for deposit in the fracture to prevent closure of the newly created drainage channels by the pressure of the overburden of earth.

Although hydraulic fracturing treatments have come into widespread use in the art of well stimulation, the fracturing process is not always successful in increasing the production of the well. The principal cause of unsuccessful fracturing operations is probably the lack of control of the direction and orientation of the fracture. Under the pressure of the hydraulic fluid the fracture is ordinarily initiated and extended along a natural plane of weakness in the formation which, of course, may extend in any direction. As a result, the fractures often extend out of the oil-bearing rock or out of the desired interval to be fractured into a gas cap above the oil in the pay zone or into Water below the oil. The low viscosity of the gas or the water as compared with the oil may result in gas and water flowing readily through the fracture to the well leaving the oil in the formation.

In attempts to control the location at which a fracture is created, several techniques have been developed. For example, either a single or dual formation or casing packer can be set in the well to isolate and confine a selected producing zone in which a fracture is desired. When casing has been set through the formation to be fractured, an opening is cut in the casing at the depth at which the fracture is desired. Either of these techniques provides control over the general location of the intersection of the fracture with the well bore but gives no control over the direction or orientation of the fracture. Underreaming a portion of the borehole has also been practiced in an attempt to induce the fracture at a desired location. By underreaming a portion of the borehole the total force available to put the rock under tension upon application of hydraulic pressure is increased. Although the ordinary type of underreaming aids in locating the fracture in the borehole wall, it is not always effective in orienting the plane of the fracture.

The present invention provides an improved method which more reliably and accurately positions and orients substantially horizontal fractures in subterranean formations. Moreover, by means of the invention, fractures are initiated at a predetermined location and orientation in subsurface formations using fluid pressures in the borehole which are lower than those normally required to initiate horizontal fractures in earth formations.

In accordance with the present invention a continuous circumferential V-shaped notch or cavity oriented in the plane of the desired fracture is mechanically cut in the borehole wall at a predetermined depth. A cutting element which is substantially greater in length than width and which tapers to a relatively sharp point at its forward edge is employed to operate on the borehole wall to form in the borehole wall a continuous circumferential V-shaped notch which has a diameter at least about twice as great as that of the borehole and a depth of penetration from the borehole surface at least about 1.3 times greater than the width of the notch at the surface of the borehole. Thereafter, hydraulic pressure is applied to the notch to fracture the formation. In a preferred mode of operation the portion of the formation in which the notch is located is isolated from the remainder of the formation before hydraulic pressure is applied to create the fracture. By forming a V-shaped notch of suitable dimensions in the borehole wall, large stress concentrations are produced at the apex of the notch, which upon application of hydraulic pressure to the notch causes the formation to fracture along the plane defined by the apex of the notch. The V-shaped notch in the borehole wall provides one continuous opening through which the hydraulic fluid passes to exert a force on the formation. The area available for flow provided by the continuous notch is large relative to the cross-sectional area of the borehole and offers minimum resistance to the introduction of large size granular propping materials. Because of the concentration of forces at the .apex of the notch, initiation of the fracture is ordinarily accomplished with much lower fluid pressure than is required with the processes heretofore available. This, of course, is an important advantage of the invention in that it lessens the requirements for high pressure pumping equipment and avoids the necessity of pressures which may damage casing or other equipment in the well.

Hydraulic pressure is applied to the notch by injecting into the well any of the fracturing fluids commonly employed in conventional hydraulic fracturing operations. These fluids may comprise hydrocarbon gels obtained by dispersing a suitable gelling agent in a hydrocarbon oil such as gasoline, kerosene or crude oil. Commonly employed gelling agents include aluminum naphthenate and mixtures thereof with aluminum oleate, hydroxy aluminum soaps of fatty acids, aluminum and other metal soaps of the various fatty acids derived from coconut oil, peanut oil and the like. Aqueous fracturing fluids which may or may not contain a thickening agent such as a natural gum, as for example, Karaya. Batu or Guar gum, which will allow the build-up of pressures sufficient to extend the fracture in the formation can also be employed. Propping materials can be employed with the fracturing fluid to maintain open the induced fracture. A preferred technique is to create and extend the fracture with a low liquid-loss liquid free of propping agent and thereafter displace a liquid containing a propping agent into the fracture. Propping materials employed can be any of the granular materials employed in the art and include, for example, glass beads, ceramic particles, metal particles, crushed rock, Wood chips, and the like. The particle size of the propping material can range from about 4 to 100 mesh U.S. sieve, but larger particles can be used, if desired.

The physical characteristics, particularly the shape and dimension of the notch created in the formation, are of the utmost importance in achieving the improved reliable control over the fracture plane. For control over the plane of the subsequent fracture, the notch created in the walls of the borehole should have a depth, that is, the distance it penetrates from the borehole, at least 1.3 times the width of the notch at the face of the borehole. Moreover, the notch should be V-shaped and have fairly regular undistorted sides which converge to form a relatively sharp apex for the notch in order to insure the concentration of forces at the confining extremity of the notch. It is particularly important to avoid two or more well defined and distinct angles, such as exist in a rectangular notch, in the outer extremity of the notch. Other requisite characteristics of the notch or cavity are that its confining extremity or apex lies in a single plane with all points on its circumference being substantially equidistant from the center of the borehole.

It has been found from experience that cutting the formation mechanically with a cutter element which tapers to a relatively sharp point at its forward edge and which has a length substantially greater than its width, is the most practical method of insuring that the desired shape of notch is obtained. Thus, while it is possible to form a circumferential notch or cavity in earth formations by setting off explosive charges within the well or by directing hydraulic jets against the formation, the mechanical cutting of a notch of the specified shape gives improved orientation of the fracture and allows the use of reduced breakdown pressures to initiate a fracture in the formation.

One preferred form of apparatus which can be employed for forming notches in the borehole wall in accordance with the present invention is described and claimed in the copending application of Jimmie L. Huitt, Joseph L. Pekarek, and Edward Topanelian, Ir., Serial No. 19,792, filed April 4, 1960, now US. Patent No. 3,050,122. This apparatus comprises a generally tubular FIGURE 1 is a vertical sectional view of an earth formation having therein a notch of the desired configuration and size.

FIGURE 2 is a vertical sectional view of the apparatus described and claimed in the above-mentioned Patent No. 3,050,122. In this view the casing severing blades and the formation notching blades are shown in position for raising and lowering in a wellbore.

FIGURE 3 is a view similar to FIGURE 2 showing the casing severing blades in a fully extended cutting position.

FIGURE 4 is a view similar to FIGURE 2 showing the formation notching blades actuated in cutting engagement with borehole walls.

In the drawings, numeral 1 designates an earth formation in which a horizontal fracture is desired. Wellbore 2 and casing 3 penetrate the earth formation 1. The notch 4 is formed in the earth formation 1 by means of the formation notching apparatus designated generally by the numeral 5. Casing severing blades 6 and formation notching blades 7 are adapted to pivot about a fixed point in the tool body and to project therefrom into cutting engagement with the casing and formation respectively. The severing of the casing and the notching of the formation is accomplished by rotating the tool at a suitable speed while the respective blades are in cutting position.

The significance of the size and geometric configuration 0f the notch created in the borehole walls with respect to controlling the fracture plane is clearly illustrated by the following fracturing tests. These fracturing tests were conducted on a number of cubes of Berea sandstone. A borehole was made through the center of each of the sandstone cubes such that the borehole diameter did not exceed A the cube dimension. The borehole was completed by mechanically cutting therein notches of various sizes and geometric configurations. The lower end of the borehole was sealed and an oil was injected into the borehole under pressure sufficient to fracture the Berea sandstone. In each case the notch formed in the Berea sandstone lay in a substantially horizontal plane as it was desired to form horizontal fractures in the sandstone. The test data are summarized in the following table:

TABLE I Ratio: Fracture Test No. Shape of notch Depth/width initiation Plane of fracture of notch pressure (a 1 Noneopen section in borehole-.. 2,200 Vertical. 2.. o 1 2,000 DO. 3 N 0ncfour dia. perforations 1 1 1, 150 Do.

in a horizontal plane. 4" n 1 1 1,000 Do. 5 Circumferential, rectangularflun, 0.5 1, 800 Do. (L 1 1,000 Approx. degrees. 7. o 2 1, 600 130. 8 Circumferential, V 0.5 1, 350 Vertical. 9. 0.5 1, 400 Do. 10 do 1 2, 000 One extensive vertical,

two minor horizontal fractures. 11 do 1 650 Vertical. 12. dn 1 1,800 One small vertical, one

small horizontal. 13. r 2 800 Horizontal. 14 do 2 950 Do. 15 2 850 Do.

1 Ratio: Depth of perforation/diameter of borehole.

housing adapted to be lowered into and rotated within the wellbore. The apparatus is provided with separate and independent elements for severing the casing and cutting the desired notch in the formation. The formation notching elements comprise a pair of cutting elements having a generally elongated V-shape and a relatively sharp point at the forward edges which when extended by hydraulic pressure project from the body of the tool into cutting engagement with the formation. The notch is cut in the formation by rotating the tool at a suitable speed while the formation notching blades project outwardly from the tool in cutting engagement with the formation.

above, fracturing of the sandstone was achieved with sig- As is seen also from the test data presented,

nificantly lower breakdown pressure when the notches were of the configuration and dimensions described herein.

The following example illustrates the horizontal fracturing of a sandy shale having a permeability of less than 100 millidarcys by the method of this invention.

Example A Well 8% inches in diameter was drilled to a total depth of 101 feet. A 5 /2 inch O.D. casing was set in the well and cemented to the surface with a 7 foot cement plug in the lower end of the casing. Observation wells were drilled feet to the east, 30 feet to the south, 40 feet to the west, and 50 feet to the north of the test well respectively. The observation wells were cased with 5 /2 inch O.D. casing to a depth of 40 feet and completed as 4% inch open holes from 40 feet to 100 feet. A formation notching apparatus having blades similar to that shown in the drawings was used to cut the casing and notch the formation at a depth of 71 feet. In the cutting of the casing the rotational speed was begun at 40 revolutions per minute and gradually increased to 60 revolutions per minute over a period of several hours. A reduction in pump pressure signaled that the casing had been cut. The width of the ring section removed from the casing was measured and found to be 4 /2 inches which was considered adequate. To notch the formation the apparatus was then positioned in the casing so as to position the formation notching blades slightly above the cut in the casing. The tool was then rotated about 40 to 50 revolutions per minute and fluid was circulated at a pump pressure of about 400 pounds per square inch gauge. This caused the formation notching blades to contact the casing slightly above the cut. The tool was then slowly lowered. As the formation blades reached the cut in the casing they expanded into the casing cut, the top face of the blades were held against the casing cut by hydraulic pressure while the tool was continuously rotated The drill string was lowered very slowly. The rate at which the tool was rotated was suflicient to maintain a smooth action rather than fast enough to cause the blades to grab. As the formation notching blades were gradually Worked into the casing cut, a reduction in pump pressure signaled that a V-notch had been cut. It was determined from examination of markings on the blades that the diameter of the continuous circumferential V-shaped notch was 17.5 inches thus indicating that the confining extremity of the notch was 6 inches from the borehole. The maximum width of the notch at the face of the borehole was 4.5 inches, thus giving a notch depth to width ratio of 1.33.

Water was pumped into the well and the pressure increased to 325 pounds per square inch at which pressure the formation fractured. The fracture in the formation was then extended by pumping into the wellbore water containing about 1 pound of sand per gallon until the fracture intersected the four observation wells. Intersection of the fracture with the observation wells was noted by water flowing from the observation wells.

To determine the depth of the fracture at each of the observation wells, air was injected into the test well. The depth at which the air reached the observation Well was determined by raising an inverted bailer from the bottom of the hole until air entering the well from the fracture collected under the bailer. The depth in the well at which air first collected under the bailer was considered to be the depth of the fracture and was measured. Using this method, it was determined that the fracture was extended to intersect the four observation wells on a horizontal plane with a deviation of only 2 feet from the horizontal at a radial distance of 50 feet. The results indicated that the V-notch had initiated the fracture in the desired plane.

A similar test was run in the same area in a well drilled to a total depth of 101 feet in which 5 /2 inch O.D. casing was set and cemented to the surface. A ring-shaped charge referred to as a 4% inch O.D. severing charge was set off at 92 feet to cut a ring-shaped section of the casing and to cut a notch in the subsurface formation.

The ring-shaped charge was followed by a four-way shaped charge at the same level. Water was pumped down the hole to initiate a fracture. Breakdown of the formation occurred at a pressure of 2500 pounds per square inch.

The numerous advantages of the present invention are manifest. The invention achieves positive control over the location and orientation of artificially induced fractures in earth formations. By means of the present invention, horizontal fractures are produced at a predetermined depth in earth formations utilizing hydraulic fluid pressure.

The fracturing method of this invention can be used in open holes as well as in cased holes. In both the open and cased holes the formation of a circumferential continuous V-shaped notch having the prescribed dimensions produces a developed plane of weakness at the apex of the notch. The subsequent application of hydraulic pressure at the notch extends the oriented plane of weakness and fixes the plane of fracture in the formation.

By the term borehole I means the hole penetrating the earth formation or in cased wells the inside diameter of the pipe which is bonded to the earth formation.

Those modifications and equivalents which fall Within the spirit of the invention and the scope of the appended claims are to be considered part of the invention.

I claim:

1. A process of creating a substantially horizontal fracture at a desired depth in a subterranean formation penetrated by a borehole of a well at an accurately controlled location, said well being filled with liquid, comprising urging into cutting engagement with the wall of the borehole at the desired depth a cutting element having a generally elongated V-shape with a relatively sharp point at its forward edge, rotating said cutting element about a vertical axis to move the relatively sharp point in a horizontal plane while in cutting engagement with the wall of the borehole while simultaneously moving said forward edge radially outward to form in the wall of the borehole a continuous horizontal circumferential V-shaped notch having a diameter at least about twice as great as that of the borehole and a depth of penetration from the borehole surface at least about 1.3 times its width at the surface of the borehole, isolating the portion of the formation in which the notch is located, and then injecting into said borehole a fluid under pressure sufiicient to fracture the formation at the location of said notch.

2. A process for creating a substantially horizontal fracture at a desired depth in a subterranean formation penetrated by the borehole -of a well containing a liquid under pressure at the desired depth, comprising moving a cutting tool having a relatively sharp point at its outer edge radially outwardly at the location of the desired fracture simultaneously with rotating said tool about a vertical axis to move said edge in a horizontal plane to cut in the formation a continuous horizontal circumferential V-shaped notch having a diameter at its apex at least about twice as great as that of the borehole and a depth of penetration at least about 1.3 times its width at the surface of the borehole, circulating a liquid in the well while cutting the notch whereby pressure is maintained at the notch and then injecting into said borehole a fluid under pressure suflicient to fracture the formation at the location of said notch.

References Cited by the Examiner UNITED STATES PATENTS 2,547,778 3/51 Reistle.

2,758,653 8/56 Desbrow.

2,988,143 6/61 Scotty.

3,050,122 8/62 Huitt et a1 166-55.8 3,066,733 12/62 Brandon.

BENJAMIN HERSH, Primary Examiner. 

2. A PROCESS FOR CREATING A SUBSTANTIALLY HORIZONTAL FRACTURE AT A DESIRED DEPTH IN A SUBTERRANEAN FORMATION PENETRATED BY THE BOREHOLE OF A WELL CONTAINING A LIQUID UNDER PRESSURE AT THE DESIRED DEPT, COMPRISING MOVING A CUTTING TOOL HAVING A RELATIVELY SHARP POINT AT ITS OUTER EDGE RADIALLY OUTWARDLY AT THE LOCATION OF THE DESIRED FRACTURE SIMULTANEOUSLY WITH ROTATING SAID TOOL ABOUT A VERTICAL AXIS TO MOVE SAID EDGE IN A HORIZONTAL PLANE TO CUT IN THE FORMATION A CONTINUOUS HORIZONTAL CIRCUMFERENTIAL V-SHAPED NOTCH HAVING A DIAMETER AT ITS APEX AT LEAST ABOUT TWICE AS GREAT AS THAT OF THE BOREHOLE AND A DEPTH OF PENETRATION AT LEAST ABOUT 1.3 TIMES ITS WIDTH AT THE SURFACE OF THE BOREHOLE, CIRCULATING A LIQUID IN THE WELL WHILE CUTTING THE NOTCH WHEREBY PRESSURE IS MAINTAINED AT THE NOTCH AND THEN INJECTING INTO SAID BOREHOLE A FLUID UNDER PRESSURE SUFFICIENT TO FRACTURE THE FORMATION AT THE LOCATION OF SAID NOTCH. 